The present invention relates to an actuator for the braking system for a vehicle, and in particular to a spring-type brake actuator.
It is well known to employ so-called “spring brake” actuators to provide service, parking and emergency brake operation on vehicles such as commercial trucks, tractors and trailers equipped with lever-operated drum or disc brakes. Spring-type brake actuators are typically pneumatically operated, and are supplied with operating air from a compressed air source on the vehicle. These actuators also typically are arranged in a “fail-safe” manner, i.e., where the actuator defaults to a brake application state upon loss of operating air pressure.
An example prior art spring brake actuator is shown in cross-section view in FIG. 1. Actuator housing 1 includes a rear cylinder 2 in which a rear piston 3 is displaceably arranged. The inner wall of the rear cylinder and a chamber-side of the rear piston define a rear ventilation chamber 4. The other side of the rear piston bears on a brake actuator spring 5. This spring is also known in the art as a “power spring” or a “parking brake spring,” and these terms may be used interchangeably. For consistency herein, the terms “brake actuator spring” or “actuator spring” will be used.
The rear ventilation chamber is isolated from the spring side of piston 3 by an annular seal 6. An intermediate flange 8 (also known as a “wall”) separates rear cylinder 2 from a front cylinder 9. The intermediate flange 8 traversed by a seal 10 through which passes a sliding rod 11, formed as an extension of rear piston 3. The sliding rod 11 can be displaced in the intermediate flange 8 by the rear piston. A front ventilation chamber 7 within front cylinder 9 is delimited by the cylinder inner wall and a front piston 13 and annular diaphragm 14. The rear piston 3 and the front piston 13 are in non-coupled contact with one another by means of the sliding rod 11, such that the front piston 13 can be displaced in a brake application direction by the rear piston 3. An actuating rod 15 for actuating a brake lever of a vehicle brake is provided on the front side of the front piston 13.
FIG. 1 also shows mounting studs 16 provided for mounting of the actuator 1 on the vehicle brake, as well as a light return spring 18 which biases front piston 13 toward the rear of front chamber 7, and a bellows seal 17 provided to keep contaminants such as brake dust from entering the portion 12 of front cylinder 9 in front of front piston 13.
When no pneumatic pressure is present in the FIG. 1 actuator unit, the brake actuation spring 5 applies a high spring force to rear piston 3, which in turn applies this force via sliding rod 11 to front piston 13 to cause the actuator rod 15 to apply the vehicle brake. In this state, the vehicle brake functions as a parking brake, preventing vehicle movement.
When release of the parking brake is desired, the rear ventilation chamber 4 is filled with compressed air via port 19. As the force generated by the increasing air pressure on the front side of rear piston 3 exceeds the force generated by brake application spring 5, the rear piston 3 and sliding rod 11 move toward the rear of the rear cylinder 2, compressing spring 5. At the same time, as sliding rod 11 moves towards the rear, the force previously applied to front piston 13 is relieved, and the return spring 18 biases the front piston 13 toward the rear of front cylinder 9, thereby withdrawing actuating rod 15 away from and releasing the vehicle brake. The vehicle therefore moves from a state in which it is braked by the brake actuator spring 5, to a non-braked state in which the vehicle may be moved. The vehicle brake is applied as a service during normal operation by admitting compressed air into the front ventilation chamber 7 (via a port not shown in FIG. 1). Because air pressure in rear ventilation chamber 4 continues to hold sliding rod 11 at the rear of the rear cylinder 2, the front piston 13 and actuating rod 15 are free to move forward and backward within the front cylinder as necessary to respond to the operator's brake actuation demands.
In the event of failure of the compressed-air supply during operation of the vehicle, the pressure in the rear ventilation chamber 4 decreases. As a result, the brake actuation spring 5 automatically pushes the rear piston 3 back to the starting (parking) position. Sliding rod 11 thus presses on the front piston 13, which in turn pushes the actuating rod 15 in the brake application direction to actuate the vehicle brake. Thus, fail-safe emergency operation of the vehicle brake is assured.
The spring brake actuators known in the prior art are complicated, are somewhat difficult to produce and service, and suffer from a number of inherent problems. First, in order to generate the very high brake application force needed to ensure full brake application in parking or emergency situations, the brake actuator spring must be powerful. As a result, brake actuator springs are large, heavy and store potentially dangerous amounts of energy when compressed. This requires that the spring brake housing to be heavily built, with relatively thick housing walls and high strength materials, to provide reliable containment of the spring and to provide an adequate foundation to absorb the reaction force of the spring as it presses against the rear end of the housing. This need is particularly acute in the case of prior art actuators, where the housing is continuously subjected to very high loads imposed by the actuator spring, and the housing must be designed to reliably withstand these loads during years of continuous exposure to harsh operating conditions. Ultimately, the need for such heavy housing construction undesirably increases the weight, size and cost of the actuator components.
A further problem with the need for the additional high strength material associated with containment of the spring is that this extra weight and the weight of the spring itself are concentrated at the least desirable location, toward the rear end of the housing. This location is not desirable because it leaves this large mass cantilevered far away from the mounting flange on the vehicle brake, maximizing the stresses placed on the actuator's mounting flanges and/or fasteners. In-service failures of mounting flanges (also known as “wing brackets”) and/or their associated fasteners have been observed which are directly attributable to stresses generated by these cantilevered masses.
Another problem with current spring-type brake actuators is the potential for injury or property damage if the brake actuator spring is not properly handled during both actuator manufacture and servicing. The typical spring brake actuator is constructed with a rear portion being detachable from the front portion of the actuator. However, because this rear portion is often the sole component retaining the brake actuator spring, great care must be taken to ensure the spring remains captured or “caged” if the rear portion is to be removed, lest the spring or the rear portion of the actuator be accelerated in an uncontrolled manner away from the housing as it is being disassembled for service. Similar concerns exist during manufacture, where the springs must be carefully controlled during actuator assembly to prevent their inadvertent escape.
The concern with potential injury or damage due to uncontrolled release of spring energy is has resulted in considerable investment in designing positive spring capture devices, technician training, and design of tamper-proof spring brake actuator housings. Nonetheless, injuries caused by improper disassembly remain a possibility. The need to provide the spring control features and fixtures for assembly and servicing also increases labor and tooling costs in both manufacturing and servicing operations, and additional cost and component weight penalties result from the need to provide robust housing flange joints and retainers (e.g., clamping mechanisms) to withstand the separating forces generated by a fully compressed brake actuator spring.
The present spring-type brake actuators are also vulnerable to internal corrosion of the spring and the rear cylinder. The rear cylinder is typically provided with at least one chamber breather on the spring side of the rear cylinder. These breathers relieve any pressure leaking into the rear of the actuator housing from the rear chamber. The corrosion concern arises from the fact that when the rear chamber is depressurized and the brake actuator spring expands back to its parking position, air entering the spring side of the cylinder through the breather contains water in the form of humidity. Rain water, road salt and de-icing solutions are also sources of corrosive water and chemicals which can enter the actuator. Corrosion from such water accumulation has led to brake actuator spring failures (e.g., fractures), defeating the “fail-safe” braking function of the actuator with little or no externally-visible warning. The water has also caused dangerous housing wall thinning, which could result in unexpected rupture of the housing, with consequent loss of emergency braking capability and the potential uncontrolled ejection of the spring and the rear piston/diaphragm.
The geometry of conventional spring brake actuators contributes to a further problem, that of jamming of the parking brake operating rod. The brake actuator spring is typically a spring coil, which is an inherently asymmetrical component which does not always provide a spring force which is perfectly aligned with the parking brake diaphragm's actuator rod. Due to the location and arrangement of the actuator spring and the parking brake rod, and the unsupported length of the rod exposed when the parking brake diaphragm is fully withdrawn, the slightly asymmetric spring force can cause the rod to lean sufficiently far to one side, dramatically increasing friction and wear of the shaft and/or its corresponding bearing surfaces and seals. If permitted to progress unchecked, such increased friction and wear could, at least theoretically, result in drag on the shaft increasing to the point that it is effectively “jammed,” i.e., unable to move out of the rear chamber when the pressure in the rear chamber is released. If this were to occur, the parking brake or emergency brake functions would not be performed by the actuator, and the “fail-safe” nature of the brake would be defeated.
In view of one or more of the foregoing problems with current spring-type brake actuators, the present invention provides an improved actuator which is safer, lighter, simpler, more reliable, less costly and/or safer to assemble and service.
The present invention eliminates the need for heavy housing structures and extra brake actuator spring capture features, by substantially rearranging the primary components of a spring brake actuator in a novel manner. In one embodiment, the brake actuator spring is relocated to the front portion of the actuator housing, occupying a position between the front service brake actuator and the rear parking brake actuator. When the spring brake actuator is inactive (i.e., no pressure exists in either the front or rear chambers), the brake actuator spring applies the vehicle brake by pressing on the service brake actuator via an intermediate spring plate, and the service brake actuator in turn presses the brake actuator rod forward in a brake application direction. The parking brake release actuator remains in the rear chamber of the actuator housing, but instead of pressing directly on the service brake actuator (as in the prior art), its attached shaft is now solidly affixed to the rear side of the intermediate spring plate. Thus, when air pressure is applied to the rear chamber, rather than compressing the brake actuator spring into the rear end of the actuator housing, as in the prior art, the present invention's parking brake release actuator draws the intermediate spring plate toward the intermediate body portion of the actuator (hereinafter, the “housing intermediate flange”), compressing the brake actuator spring against the front side (or “floor”) of the intermediate flange to remove the spring's force from the actuator rod. This arrangement preserves the “fail-safe” nature of the spring-type brake actuator (i.e., loss of pressure in the rear chamber still results in the brake actuator spring re-applying the brake), while also positively capturing the spring between the spring plate and the intermediate flange.
The present invention offers a number of significant advantages over previous spring brake actuator designs, stemming primarily from the decreased structural requirements on the housing, and the inherent self-capture of the brake actuator spring.
First, because the brake actuator spring is located such that its reaction force is absorbed by the actuator housing's intermediate flange rather than the rear portion of the housing structure, the rear portion now must withstand only the pressure applied to the parking brake actuator (e.g., the pneumatic pressure applied to an actuator diaphragm or piston). This change means that there is no longer any need for thick-section castings or high strength materials to withstand the high tensile loads imposed by a rear-mounted brake actuator spring. The rear housing structure therefore may be designed and built much lighter, and potentially smaller, than previously possible. For example, in place of previous high-strength alloy castings or steel-based housings, lighter structures may be used, such as simple cast aluminum, molded plastic or composite components. In addition to weight savings, use of simpler, lighter rear housing components offers additional cost and corrosion resistance advantages.
Further, due to the much lower head and hoop stresses in the rear housing structure, the joint between the housing intermediate flange and the rear portion of the housing may be made much lighter and simpler while still providing satisfactory sealing and retention of the rear portion of the housing. For example, simple roll crimps or adhesives may potentially be used where previously heavy flanges and thick retaining bands were required. The reduced stresses also provide the opportunity to increase actuator serviceability. Previously, considerable engineering and production effort was invested in designing and producing rear housing joints which were so-called “tamper-proof” joints, in order to discourage improper disassembly of a rear housing in which an uncaged spring was present. Elimination of the actuator spring loads and associated stresses from the rear housing joint eliminates any need for a “tamper-proof” joint, thus permitting the joint to be designed to be released and re-made as needed.
The potential reduction in actuator size resulting from the reduced structural loads and component relocations is also a significant advantage, as space within, and immediately adjacent to, a commercial vehicle wheel rim is at a premium, and will only become more scarce as air-operated commercial vehicle disc brakes and other new brake technologies begin to displace older drum-style brakes.
The present invention also permits reduction in the structure required to mount and support the spring brake actuator on a vehicle brake, as well as enhancing the reliability of the mounting structures. In the past, heavy masses cantilevered far from the mounting surface at the rear of the actuator (i.e., the rear cylinder and brake actuator spring) caused stresses at the mounting flanges and required fasteners having a robust structure. Even with such designs, the mounts were known to occasionally fail. By moving the weight of the brake actuator spring much closer to the mounting flange and eliminating excess weight from the rear housing structure, the loads on the mounting flange and its fasteners are greatly reduced. The mounts therefore may also be redesigned to reduce their size and weight, while continuing to maintain or even increase the reliability of the mounting system.
Another advantage of the present invention's arrangements is the increase in safety afforded during production and servicing operations. With previous spring brake actuators, there was a constant danger of the spring violently escaping the spring brake housing or propelling a portion of the housing toward a technician if proper assembly or disassembly procedures were not followed or if the housing ruptured. By locating the brake actuator spring and the parking brake release actuator on opposite sides of the housing's intermediate flange and then linking these elements together via the intermediate spring plate, the parking brake release actuator positively captures the spring at all times, eliminating the spring release danger. Moreover, this arrangement also minimizes any danger of launching either the rear portion of the housing or the intermediate flange, because once the parking brake release actuator is resting on the intermediate flange (which occurs every time the spring brake is deactivated), further extension of the brake actuating spring is precluded.
The positive capture of the brake actuator spring in the present invention also improves assembly and servicing operations, by allowing both the front and rear portions of the actuator housing to be assembled to, or removed from, the intermediate flange without high spring pressure loads acting against ends of the housing. This permits expendable components within the spring brake actuator to be accessed and replaced with a much lower risk of injury, and in less time than with previous spring brake actuators. Such improvements offer corresponding decreases in assembly and servicing costs.
In further embodiments of the present invention, additional production and servicing advantages may be realized. The parking brake release actuator, its shaft and the intermediate spring plate may be designed in a manner which allows all the pre-load on the spring to be released before the spring is released from the housing's intermediate flange. For example, the intermediate spring plate could be affixed to the parking brake release actuator shaft by a bolt threaded into the center of the shaft. The shaft and the bolt could be made with sufficient length, such that by the time the bolt reaches the end of its engagement with the shaft, the intermediate spring plate and the parking brake actuator are so apart enough that the free length of the spring has been exceeded, removing all pre-load on the spring. The unloaded spring plate could then be removed to, for example, allow the shaft to be extracted for replacement of a seal in the intermediate flange. Alternatively, the spring plate could be welded to the shaft, and the parking brake release actuator be threadably engaged with the shaft on the rear cylinder side of the intermediate flange. Alternative fastening variations which accomplish the objective of unloading the brake actuator spring pre-load will be apparent to those of ordinary skill in the art.
A further advantage of the present invention is the opportunity to eliminate any need for ventilation of the volume in which the brake actuator spring is located, which in turn eliminates the primary source of spring corrosion and spring failure. The present invention therefore offers improved long-term spring brake actuator reliability. In previous spring brake actuator designs, it has been common to provide a vent to atmosphere from the spring side of the rear cylinder, in order to prevent pressure leaking from the rear chamber toward the spring from building up to the point of rendering the parking brake release actuator ineffective (and thereby preclude brake release). In the present invention, because the brake actuator spring has been removed from the chamber containing the parking brake release actuator, there is no need for a breather valve in the vicinity of the brake actuator spring. Corrosion protection may be further enhanced by eliminating essentially any moisture-bearing air exchange between the region around the spring and the rest of the front chamber by providing a simple seal between the outer rim of intermediate spring plate and the inner wall of the front chamber. This aspect of the present invention also contributes to simplification of the design and lower cost by eliminating unnecessary parts (the breather and associated fittings, filters, etc.) and the design and tooling costs associated with machining of housing components to accept the breather and related fittings.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.